Method and apparatus for decoding received data in a communications system

ABSTRACT

A plurality of data streams, which are coded by a convolution code and where coding rates and modulation systems are set respectively, and multiplexed in time division so that multiplex data are composed. A signal-to-noise ratio monitor measures the strength of the noise included in the multiplex data. A comparator outputs a post signal when a comparison indicates that the measured noise strength is equal to greater than a preset value. A multiplex control signal generator inputs an initialization signal into a Viterbi decoder at a timing at which decoding of individual data streams is started based on the received post signal. The Viterbi decoder initializes the calculated path metric upon receipt of the initialization signal.

FIELD OF THE INVENTION

The present invention relates to a technology of demodulating a receiveddigital-modulated signal and decoding actual data from digital dataobtained by the demodulation. More particularly, the invention relatesto a decoding method, a decoder, a data receiving system, and a datatransmission and receiving system which decodes a data stream, which iscoded by using a convolutional code and is multiplexed in time division,using Viterbi decoding.

BACKGROUND OF THE INVENTION

Recently, as integrated circuits and coding techniques, which canprocess digital data at a high speed, are becoming available. Therefore,analog-system techniques are now replaced by such digital techniques.Particularly, growth television broadcasting service, satellitebroadcasting service and the like are being converted to digitalbroadcasting.

The digital satellite broadcasting adopts a radio communication systemin which a carrier signal is transmitted via an artificial satellite andis received directly by a receiver installed in a house or the like.However, since this digital satellite broadcasting utilizes theatmosphere as a transmission space, it is easily influenced by weatherand the other factors. When the weather is bad the reception becomesworse. As a result, in a transmitter and a receiver which are terminalsections of the digital satellite broadcasting system, thetransmission/receiving process with high stability and high reliabilityis required in comparison with the conventional cable and ground wavetelevision service.

In radio communication of a digital-modulated signal like the digitalsatellite broadcasting, digital data to be digital-modulated arecomposed of bit strings (data stream) which undergo a so-called codingprocess in which redundancy is added to actual data so that thestability and reliability of the transmission/receiving process isheightened.

As the redundancy of the data stream is higher, error correction abilityof the data stream is improved further in the decoding process in thereceiver. In other words, even if the data stream with high redundancyinclude a lot of error bits upon receiving, the error bits can becorrected and the data stream can be reproduced correctly upontransmission. Meanwhile, such data stream with high redundancy has adisadvantage that a number of structure bits increases and transmissionefficiency is lowered.

When image or sound information is received as data stream including alittle error, for example, the information as a whole can be accepted.Therefore, the redundant may be comparatively lowered and thetransmission efficiency heightened so that a lot of information can betransmitted. On the other hand, for a data stream which representscomputer programs where even one bit error is not allowed, generally theredundancy is set to comparatively higher value because upon receivingthe information is required to be securely restored.

FIG. 1 is a block diagram showing a schematic structure of aconventional transmitter and particularly shows a portion of thetransmitter used in digital satellite broadcasting. This transmitter iscomposed of a coder 100 and a modulator 150. The coder 100 multiplexesand codes data from a plurality of information sources 1, 2, . . . , n.The modulator 150 modulates the coded signals according to apredetermined modulation system. The information sources here aredigital data strings (transport stream) which are compressed by MPEG2(Motion Picture Expert Group 2) as one of the motion picture compressingsystems.

Further, the coder 100 is composed of a multiplexer 110, a convolutioncoder 120, a punctured module 130 and a multiplex control signalgenerator 140. Although not shown in the figure, the multiplexer 110 iscomposed of, for example, a Reed-Solomon coding circuit, a framestructure circuit, an energy dispersion circuit and an interleaver andmultiplexes and codes the input information sources. Operation of thetransmitter is explained below.

The multiplex control signal generator 140 generates a multiplex controlsignal which represents multiplexed information of the informationsources. The multiplexed information is the information which representspositions (timing) of information source data multiplexed on one carrierand transmission systems (redundancy, modulation systems, etc).

The information sources are input into the Reed-Solomon coding circuitof the multiplexer 110. The Reed-Solomon coding circuit adds aReed-Solomon code which, can correct an error of a byte unit in thereceiver based on the multiplex information represented by the multiplexcontrol signal output from the multiplex control signal generator 140,to the bit strings of the information sources so as to output the codedstrings of the respective information sources.

The code strings output from the Reed-Solomon coding circuit are inputinto the frame structure circuit. The frame structure circuitmultiplexes the code strings based on the multiplex informationrepresented by the multiplex control signal output from the multiplexcontrol signal generator 140 so as to structure a frame to be a unit ofthe multiplexed data.

A signal output in the frame unit from the frame structure circuit isinput into the energy dispersion circuit. The energy dispersion circuitadds (scramble) a pseudo random signal (energy dispersion signal) to thedigital data. As a result, the digital data composing the input frames,namely, bit strings are not transmitted as enumeration of bit “0” or bit“1” for long period.

The aim is to prevent misconception of reception such as error detectionor non-detection of a digital signal on the receiving side due toreception of the long-period continuous same bits. The pseudo randomsignal should be removed on the receiving side. For this reason, also inthe energy dispersion circuit, a position of the digital data, whereinformation representing a generating condition of the pseudo randomsignal such as a random initial value and the like is shown, isdetermined by referring to the multiplex information.

The signal scrambled in the energy dispersion circuit is input into theinterleaver. The interleaver rearranges the digital data represented bythe input signal in byte unit so as to improve resistance to a bursterror (long-period continuous error) appearing intensively in time.

As a result, even if burst errors occur in the rearranged digitalsignals, the errors which occur intensively can be dispersed because theprocess for restoring the rearrangement of the digital signals (theprocess by the interleaver) is executed on the receiving side. As aresult, improvement of the error correction and a correct recognitionrate of the transmitted information can be heightened. The informationabout the rearrangement can be obtained also from the multiplexinformation.

The signals arranged by the interleaver are the outputs of themultiplexer 110 and are input into the convolution coder 120. Theconvolution coder 120 executes the convolution coding process withrespect to the input signals. As a result, random errors such as errorsof irregular bit units such as a thermal noise which are generated in atransmission line or the receiver can be corrected.

FIG. 2 is a block diagram showing a schematic structure of theconvolution coder 120. The convolution coder 120 is composed of a shiftregister comprising D latches 121 and 122, two EXOR circuits 124 and125, and a parallel-serial converter 128. The convolution coder 120executes the coding in such a manner that one-bit input data are outputas 2-bit data. When the coding rate is defined as (Original informationcontent)/(coded information content), the coding rate in the convolutioncoder 120 is 1/2.

In the convolution coder 120 shown in FIG. 2, two-bit serial data areheld by the D latches 121 and 122 in the order of input. Three-bitserial data are converted into parallel data by the two-bit data andone-bit data further input. Newly input data and the data held in the Dlatches 121 and 122 are input into the EXOR circuit 124 so as to undergoexclusive OR. Moreover, the newly input data and the data held in the Dlatch 122 are input into the EXOR circuit 125 so as to undergo exclusiveOR.

The calculated results of the two exclusive OR are again converted intoserial data in the parallel-serial converter 128. As a result, three-bitinput serial data are output as six-bit serial data, for example, andthis output result becomes a convolution code.

In order to improve the transmission efficiency, four-bit data, whichare obtained by thinning out two bits from six-bit data output from theconvolution coder 120, are used as output data. In this method, thecoding rate is 3/4, and in comparison with the coding rate 1/2 in thecase of the convolution coder 120, redundancy can be made lower and thetransmission efficiency can be heightened.

Furthermore, this method simultaneously reduces the error correctionability. As a result, the redundancy can be controlled by changing adegree of the thinning-out of data. This data thinning-out process iscalled as puncturing. The punctured module 130 shown in FIG. 1 executessuch puncturing.

FIG. 3 is an explanatory diagram showing an example of the coding rateobtained by puncturing. FIG. 3 shows puncturing of the coding rates 2/3,3/4, 5/6 and 7/8 which is generated by thinning out a convolution codeof the coding rate 1/2. Explanation is provided below for the puncturingof the coding rate 3/4.

It is assumed that data “x0, y0, x1, y1, x2, y2” are obtained from dataconstituted by “d0, d1, d2” by the convolution coder 120. In such acase, the punctured module 130 deletes a bit in a position correspondingto “0” with reference to a previously prepared bit deletion map “1, 1,0, 1, 1, 0”. Undeleted bit after the deleted one is shifted to theposition of the deleted bit. In other wards, this bit deletion map showsthat two bits are thinned out from six bits. As a result, only four-bitdata of “x0, y0, y1, x2” are output.

Thus, the punctured module 130 enables changes of various kinds in thecoding rates as shown in FIG. 3. When a various kinds of bit deletionmaps are prepared, the transmission efficiency can be selected accordingto a signal transmission line or a signal characteristic.

The code strings output from the punctured module 130 in such a mannerbecome a data stream which is obtained by coding and multiplexing dataof a plurality of information sources with the coding rates determinedin the respective information source. The data stream is output from thecoder 100. The data stream is input into the modulator 150 shown in FIG.1 and is digital-modulated according to a modulation system which issuitable to the carrier so as to be output as a transmission signal.

As the digital modulation executed in the modulator 150, modulationslike amplitude modulation (ASK), frequency modulation (FSK), phasemodulation (PSK) may be considered. An explanation is provided below forthe digital phase modulation.

The digital phase modulation is a system where the bit structurescomposed of “0” or “1” of the digital data are made to havecorrespondence to phases, and the phases are changed for the carrier sothat information is transmitted. The digital phase modulation systemfurther includes BPSK, QPSK (or 4PSK), 8PSK and the like according to anumber of phases to be used.

As for the transmission signal which is digital-phase modulated, onephase state (transmission symbol) of the carrier is checked in thereceiver so that one-bit information can be transmitted in BPSK andtwo-bit information in QPSK and three-bit information in 8PSK. Thisshows that the transmission efficiency varies with the respective phasemodulation systems. However, as the transmission efficiency becomeshigher, adjacent transmission symbols are closer to each other so thatclear distinction between the phases becomes difficult. As a result,error can be easily generated in the information. For this reason, thephase modulation using these three systems is selected according to acharacteristic of information to be transmitted.

In other words, as for the data of the information sources multiplexedas the data stream, in addition to the selection of the coding rates bymeans of the coder 100, the modulation system by means of the modulator150 can be selected.

FIG. 4 is an explanatory diagram showing a structure of the data streamoutput from the coder 100. The data stream shown in FIG. 4 is generatedby the frame structure circuit. A data stream 1, a data stream 2, and adata stream 3 which represent three information source data aremultiplexed and arranged in a frame identified by a synchronous code.For example, the data stream 1, the data stream 2, and the data stream 3can be allocated in this order to a QPSK modulation stream of the codingrate 3/4, a QPSK modulation stream of the coding rate 1/2 and a BPSKmodulation stream of the coding rate 1/2.

In addition, some frames can be processed as one collective information(hereinafter, referred to as “super frame”). In this case, in the superframe composed of eight frames, for example, the synchronous code isarranged at the head of each frame, and a parity signal corresponding tothe Reed-Solomon code is arranged in the last two frames so that theerror correction ability is heightened.

An explanation is provided below for a conventional receiver whichreceives a transmission signal transmitted from the transmitter anddemodulates and decodes the signal. FIG. 5 is a block diagram showing aschematic structure of such a conventional receiver. FIG. 5 shows oneexample of the receiver of the digital satellite broadcasting which issuitable to the transmitter in FIG. 1. This receiver is composed of ademodulator 190 further having a digital phase modulation circuit and adecoder 200.

The decoder 200 is composed of a depuncture module 210, a Viterbidecoder 220, a synchronizer 230, a data stream decoder 240, a multiplexcontrol signal generator 250, and a multiplex information storagesection 260. The data stream decoder 240 is composed of, for example, adeinterleaver, an energy dispersion signal removal circuit and aReed-Solomon code error correcting circuit correspondingly to thesimilar structure in the multiplexer 110 shown in FIG. 1. The datastream decoder 240 decodes the multiplexed data stream. Operation ofthis receiver is explained below.

Similarly to the multiplex control signal generator 140 shown in FIG. 1,a multiplex control signal is generated in the multiplex control signalgenerator 250 of the decoder 200. Multiplex information represented bythe multiplex control signal is previously stored in the multiplexinformation storage section 260. The multiplex control signal generator250 generates a multiplex control signal based on the multiplexinformation.

The demodulator 190 demodulates the received signal in accordance withacquisition timing of the information source data represented by themultiplex control signal and the modulation system. Precisely, thedemodulator 190 extracts a code string in the state before the receivedsignal is modulated in the modulator 150 of the transmitter, and digitalphase demodulation is executed in this example.

The signal demodulated by the demodulator 190 is input into thedepuncture module 210 of the decoder 200. The depuncture module 210inserts the bit which is deleted in the punctured module 130 into theinput signal based on the coding rate represented by the multiplexcontrol signal (depuncturing).

The code string which is depunctured by the depuncture module 210 isinput into the Viterbi decoder 220 so that the convolutional code codedin the convolution coder 120 of the transmitter is decoded. That is, theViterbi decoder 220 calculates a Hamming distance between the coderepresented by the input code string and a code on a trellis chart aspath metric and leaves path metric of short Hamming distance as survivalpath. Further, the Viterbi decoder 220 decodes a code stringcorresponding to a path metric of the shortest Hamming distance as amaximum code.

The code string decoded by the Viterbi decoder 220 is input into thesynchronizer 230. The synchronizer 230 detects a synchronous code in theframe shown in FIG. 4 from the data stream and generates a controlsignal for acquiring the decoding timing of the multiplexed codestrings.

FIG. 6 is a block diagram showing a schematic structure of thesynchronizer 230. As shown in FIG. 6, the received code string is inputinto a sync word detection circuit 231 and a buffer 235. The sync worddetection circuit 231 detects the synchronous code from the input codestring and generates a detection signal. The synchronous acquisitioncontrol circuit 232 outputs a signal representing synchronous timingaccording to the received the detection signal and a predeterminedsynchronous clock.

A control signal generation circuit 233 inputs the signal output fromthe synchronous acquisition control circuit 232 so as to output acontrol signal representing that current time is positioned at the headof the frame. Meanwhile, the code string input into the buffer 235 isdelayed until output of the control signal is completed so as to beoutput as data stream at predetermined timing. The control signal outputfrom the synchronizer 230 is input into the multiplex control signalgenerator 250 shown in FIG. 5 so as to be utilized for obtaining outputtiming of the multiplex control signal.

The data stream output from the synchronizer 230 is input into the datastream decoder 240. The data stream then undergoes the decodingprocesses corresponding to the coding processes in the interleaver, theenergy dispersion circuit and the Reed-Solomon coding circuit composingthe multiplexer 110 shown in FIG. 1.

The decoded data bit string output from the data stream decoder 240becomes an output of the decoder 200 and is input into a not shown MPEGreproduction apparatus or the like, which is connected in the laterstage. This MPEG reproduction apparatus extracts corresponding data fromthe data bit string by selecting an information source, and displays theextracted data as a motion picture.

However, as mentioned above, the transmission-reception system composedof the convolution coder 120 and the Viterbi decoder 220, coding anddecoding are executed by the calculation methods based on superposing ofpast bit strings continuously transmitted. For this reason, a datastream having coding rate of high error correction ability is influencedby the case that error cannot be corrected in a data stream havingcoding rate of low error correction ability. As a result, the wholeerror correction ability of the multiplexed data stream is lowered.

For example, consider a case in which two or more kinds of coding ratesare used between the data streams composing the multiplexed data stream,and comparatively big noise is mixed on the transmission line. In thiscase, although error can be sufficiently corrected in the data streamhaving coding rate of high error correction ability in the multiplexeddata streams, error is still included in a decoded result of the datastream having coding rate of low error correction ability. In thisstate, the Viterbi decoder decodes the data stream having coding rate ofhigh error correction ability which is next input continuously accordingto calculation of path metric using the data stream including the error.For this reason, the data stream having coding rate of high errorcorrection ability cannot be decoded correctly.

In order to solve such a problem, Japanese Patent Application Laid-OpenNo. 9-247003 discloses “information receiver”. This information receiverinserts a known “end bit” into a plurality of multiplexed data streamsby means of the convolution coder just before the coding rate changes.The Viterbi decoder initializes path metric at timing that the “end bit”is detected.

As a result, in the state that the path metric is initialized per datastream, namely in the state that a storage device of a Hamming distancecalculated in the Viterbi decoder is initialized, Viterbi decoding canbe started. This prevents the data streams having different coding ratesfrom being influenced by the decoding.

However, the “information receiver” cannot solve the above problem whenthe “end bit” cannot be inserted into the data streams from theviewpoint of the standard of data transmission in digital satellitebroadcasting or the like. If the “end bit” is not inserted and notdetected and the path metric is initialized at a change point of thecoding rate, namely, a point that transmission of next data stream isstarted, there arises a new problem that a convolutional code fails andthe error correction ability is lowered.

For example, in a data stream which is obtained by multiplexing a codeof low error correction ability and a code of high error correctionability, an error of a code having low error correction ability whichcan be corrected sufficiently occurs on a transmission line. In thiscase, when the path metric is initialized at the change point of thedata stream, a vicinity of the head of the data stream cannot bedecoded. As a result, more errors occur than the case that theinitialization is not executed.

SUMMARY OF THE INVENTION

It is an object of the invention to initialize path metric in a Viterbidecoder according to strength of a noise on a transmission line andcoding rates of continuous data streams so as to optimally decode aplurality of data stream, which are multiplexed without inserting an“end bit” thereinto, with high stability and reliability.

In order to solve the above problem and achieve the above object, in adecoding method and a decoder of the present invention a signal-to-noiseratio monitor measures the strength of the noise included in themultiplex data, and a comparison unit outputs a post signal when themeasured noise strength is equal to or greater than a preset value, andan initialization signal generation unit inputs an initialization signalinto the Viterbi decoder at timing that decoding of the data streams isstarted according to the receipt of the post signal so that calculatedpath metric is initialized.

Thus, only when the noise which exceeds a predetermined value isdetected by the signal-to-noise ratio monitor, the path metric of theViterbi decoder is initialized. For this reason, only in the case wherea noise where an initializing effect can be obtained even after addingdeterioration of the decoding characteristic due to the initializationoccurs, the initialization is possible.

Further, a signal selection unit initializes path metric of the Viterbidecoder only when a coding rate of a data stream to be decode is largerthan a coding rate of a data stream which has just been decoded.

Thus, only when the timing that the initialization signal is input isthe point that the data stream having coding rate of low errorcorrection ability is changed into the data stream having coding rate ofhigh error correction ability, the Viterbi decoder is initialized. Forthis reason, the calculated result of the path metric calculated bydecoding the data stream having coding rate of high error correctionability can be utilized when the next data stream having coding rate oflow error correction ability is decoded.

Other objects and features of this invention will become apparent fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing a schematic structure of aconventional transmitter.

FIG. 2 is a block diagram showing a schematic structure of a convolutioncoder in the conventional transmitter.

FIG. 3 is an explanatory diagram showing an example of a coding rateobtained by puncturing in the conventional transmitter.

FIG. 4 is an explanatory diagram showing a structure of a data streamoutput from a conventional coder.

FIG. 5 is a block diagram showing a schematic structure of aconventional receiver.

FIG. 6 is a block diagram showing a schematic structure of asynchronizer in the conventional receiver.

FIG. 7 is a block diagram showing a schematic structure of a decoderaccording to a first embodiment.

FIG. 8 is a block diagram showing another schematic structure of thedecoder according to the first embodiment.

FIG. 9 is a block diagram showing a schematic structure of the decoderaccording to a second embodiment.

FIG. 10 is a block diagram showing a schematic structure of the decoderaccording to a third embodiment.

FIG. 11 is an explanatory diagram showing a structure of a data streamwhich is decoded in the decoder according to a fourth embodiment.

FIG. 12 is a block diagram showing a schematic structure of the decoderaccording to the fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a decoding method and a decoder of the presentinvention are explained below with reference to the attached drawings.However, this invention is not limited to these embodiments.

A decoding method and a decoder according to a first embodiment isexplained here. The decoding method and the decoder according to thefirst embodiment have the following characteristic in addition to thecharacteristic of the conventional decoder 200. Precisely, noisestrength of a signal output from the demodulator 190 is measured andwhen the measured noise strength exceeds a predetermined value only thenan initialization signal which instructs initialization of path metricis input into the Viterbi decoder 220 via the multiplex control signalgenerator 250. As a result, decoding can be executed optimally inaccordance with the strength of the noise mixed on the transmissionline.

FIG. 7 is a block diagram showing a schematic structure of the decoderaccording to the first embodiment. The decoder 10 shown in FIG. 7 isprovided with a signal-to-noise ratio monitor 12 and a comparator 14.The signal-to-noise ratio monitor 12 measures strength of noise from asignal input into the depuncture module 210 so as to digitize themeasured noise strength. The comparator 14 compares the output of thesignal-to-noise ratio monitor 12 with a predetermined value and outputsa signal representing the result of comparison (hereinafter, referred toas a post signal). The multiplex control signal generator 250 outputs aninitialization signal to the Viterbi decoder 220 based on the postsignal received from the comparator 14. This process is different fromthe one executed in the conventional decoder 200.

The legends same as in FIG. 5 are provided to the other components whichare common to those of the conventional decoder 200, and the explanationthereof is omitted. The Viterbi decoder 220 initializes an internalHamming distance storage device according to an external signal.

Operation of the decoder 10 will be now explained. A signal output fromthe demodulator 190 or the like shown in FIG. 5 is input into thedepuncture module 210 and the signal-to-noise ratio monitor 12. Thesignal-to-noise ratio monitor 12 measures noise strength of BER (BitError Rate) or the like with predetermined time intervals from the inputsignal so as to output the measured result as a measurement signal. Themeasurement signal output from the signal-to-noise ratio monitor 12 isinput into the comparator 14 so as to be judged as to whether or not anoise is allowable.

It is assumed that a value which represents an upper limit of theallowable noise has been previously set in the comparator 14. When thevalue of the measurement signal exceeds this set value, a low-levelpulse signal is output as a post signal, for example. Moreover, when thevalue of the measurement signal does not exceed the set value, a signalin a high-level state is output.

Meanwhile, the signal input into the decoder 10 is input into thedepuncture module 210, the Viterbi decoder 220, the synchronizer 230 andthe data stream decoder 240 successively so as to undergo the decodingprocesses. A control signal generated by the synchronizer 230 is fedback to the multiplex control signal generator 250. The control signalis used for taking timing of a multiplex control signal which decodesrespective data streams according to multiplex information in thedepuncture module 210 and the data stream decoder 240.

The multiplex control signal generator 250 receives the post signaloutput from the comparator 14 thereinto. When the post signal representsthe state that the value exceeds the set value, namely, a big noise ismixed in the received signal, the initialization signal is output attiming based on the control signal output from the synchronizer 230,namely, timing that decoding of the respective data streams is started.

The initialization signal is input into the Viterbi decoder 220. TheViterbi decoder 220 initializes Hamming distances which have beencalculated and stored, namely, path metrics according to the input ofthe initialization signal. As a result, an influence of the superposednoise in the decoding of the code string in the previous position can beavoided.

There arises a problem here that reliability of the maximum judgment ofViterbi decoding is lowered at the first stage of the data streambecause the path metric is initialized. However, in the state that anoise of not less than certain strength is mixed in the received signal,it was confirmed by an experiment conducted by the inventors that moreaccurate decoding can be executed by such initialization process.Therefore, it is necessary to set the set value in the comparator 14 asa threshold value as to whether or not an effect is produced when theinitializing process is carried out.

The set value in the comparator 14 can be changed with an externalsignal by providing a register. FIG. 8 is a diagram showing a schematicstructure in the case where the register is provided in the decoderaccording to the first embodiment. The register 22 stores and holds theset value. The comparator 14 refers to the set value stored in theregister 22 so as to be capable of changing the post signal to be inputinto the multiplex control signal generator 250, namely, an initializingcondition according to the a condition of the transmission line and acode to be received.

In the decoders 10 and 20, the signal-to-noise ratio monitor 12 maymeasure noise strength for a modulated signal output from the modulator,not shown. Further, the signal-to-noise ratio monitor 12 may measurenoise strength for a received signal to be input into the demodulator.

Further, an initialization signal generator may be provided instead ofthe multiplex control signal generator 250 to generate theinitialization signal.

The decoding method and the decoder according to the first embodimentmeasures the strength of noise mixed in the received signal in thetransmission system which transmits and receives data composed ofmultiplexed data stream. When the noise has a value which is equal to orgreater than the predetermined set value, the initialization signalrepresenting the initialization of the path metric is input into theViterbi decoder 220 at a predetermined timing. For this reason, thedecoded result with high reliability can be obtained in comparison withthe decoded result obtained in the case where a comparatively big noiseis mixed in the received signal.

Particularly according to the decoding method and the decoder of thefirst embodiment, in the case where a noise that error cannot becorrected sufficiently at the low coding rate is generated, securedecoding with originally high error correction ability can be executedin the data stream having the high coding rate without being influencedby the noise.

A decoding method and a decoder according to a second embodiment will benow explained. The decoding method and the decoder according to thesecond embodiment have the following characteristic in addition to thecharacteristic of the decoder 20 according to the first embodiment.Precisely, the initialization signal is input into the Viterbi decoder220 only at a point that the data stream having coding rate of low errorcorrection ability is changed into the data stream having coding rate ofhigh error correction ability.

FIG. 9 is a block diagram showing a schematic structure of the decoderaccording to the second embodiment. The decoder 30 is provided with asignal selector 32. The signal selector 32 outputs the initializationsignal to the Viterbi decoder 220 based on the multiplex informationstored in the multiplex information storage section 260. The presenceand absence of this signal selector 32 is the difference between thedecoder 20 shown in FIG. 8 and the decoder 30. Legends same as in FIG. 8are provided to the other components which are common to those of thedecoder 20 according to the first embodiment, and the explanationthereof is omitted.

Operation of the decoder 30 will be explained below by taking intoconsideration only the differences with respect to the first embodiment.In FIG. 9, it is assumed that a post signal which represents that thenoise exceeds the set value is output by the comparator 14 to themultiplex control signal generator 250. The multiplex control signalgenerator 250 outputs the initialization signal at a timing of thecontrol signal output by the synchronizer 230. The initialization signalis input into the signal selector 32. The signal selector 32 alsoreceives the multiplex information stored in the multiplex informationstorage section 260. The signal selector 32 then judges as to the timingthat the initialization signal is input shows a point that which datastream is changed into which data stream.

Particularly, when the signal selector 32 judges that the timing thatthe initialization signal is input is the point that the data streamhaving coding rate of low error correction ability is changed into thedata stream having coding rate of high error correction ability, onlythen the initialization signal is allowed to pass to the Viterbi decoder220.

In the decoding of the data stream having coding rate of high errorcorrection ability, an error can be corrected sufficiently even in asignal into which a comparatively big noise is mixed. As a result,coding of the next data stream having coding rate of low errorcorrection ability is not influenced. On the contrary, if theinitialization is executed at this change point, the decodingcharacteristic is deteriorated.

The signal selector 32 is provided so as to prohibit the Viterbi decoder220 from being initialized at the point that the data stream havingcoding rate of high error correction ability is changed into the datastream having coding rate of low error correction ability.

The decoding method and the decoder according to the second embodimentinitialize the Viterbi decoder 220 in the transmission system whichtransmits and receives data composed of multiplexed data stream at thepoint that the strength of noise mixed in the received signal exceedsthe predetermined value and the data stream having coding rate of lowerror correction ability is changed into the data stream having codingrate of high error correction ability. As a result, the coded resultwith high reliability which is not much influenced by the noise can beobtained.

A decoding method and a decoder according to a third embodiment will benow explained. The decoding method and the decoder according to thethird embodiment have the following characteristic in addition to thecharacteristic of the decoder 30 according to the second embodiment.Precisely, a plurality of multiplexed data stream are extracted from thedata stream decoded by the data stream decoder 240 so as to bedistributed.

FIG. 10 is a block diagram showing a schematic structure of the decoderaccording to the third embodiment. This decoder 40 is provided with adistribution device 42, which distributes the plurality of data streamcomposing the data stream, at the later stage of the data stream decoder240. This point is different from the decoder 30 shown in FIG. 9.Legends same as in FIG. 9 are provided to the other components which arecommon to those of the decoder 30 according to the second embodiment,and the explanation thereof is omitted. The multiplex control signalgenerator 250, in addition to other functions, generates a signalrepresenting distribution timing of the data streams by means of thedistribution device 42.

The multiplexed data streams are selected from the data stream decodedby the data stream decoder 240 so as to be distributed in the apparatusto which the decoders 10, 20 and 30 are connected. In this case, thisapparatus to which the decoders are connected has to hold multiplexinformation and generate a multiplex control signal by means of thecircuit configuration corresponding to the multiplex control signalgenerator 250 and the multiplex information storage section 260.

The distribution device 42 is provided at the later stage of the datastream decoder 240 in the decoder 40 so that the circuit configurationof the whole receiver equipped with this decoder 40 can be simplified.Signals which change punctured codes and the like already exist in themultiplex control signal generator 250 or the like in the decoder 40.Therefore, it is easy to generate a signal which distributesinformation.

The decoding method and the decoder according to the third embodimentinitialize the Viterbi decoder 220 in the transmission system whichtransmits and receives data composed of multiplexed data stream at thepoint that the strength of noise mixed in the received signal exceedsthe predetermined value and the data stream having coding rate of lowerror correction ability is changed into the data stream having codingrate of high error correction ability. Further, the distribution device42 which distributes multiplexed data streams is provided at the laterstage of the data stream decoder 240. As a result, the effect of thesecond embodiment can be produced, and the circuit configuration of thewhole receiver equipped with the decoder 40 can be simplified.

A decoding method and a decoder according to a fourth embodiment will benow explained. In the decoders 10, 20, 30 and 40 according to the firstto third embodiments, the multiplex information is known in thereceiving side and is previously stored in the multiplex informationstorage section 260. However, the decoding method and the decoderaccording to the fourth embodiment is characterized in that themultiplex information is included in the transmission signal transmittedfrom the transmitter and the decoding process is executed dynamicallyupon receiving the multiplex information.

FIG. 11 is an explanatory diagram showing a structure of the data streamwhich is decoded by the decoder according to the fourth embodiment. Asshown in FIG. 11, as for the signal which is input into the decoderaccording to the fourth embodiment via the demodulator, not shown, themultiplex information detected by a synchronous code 1 and themultiplexed data streams 1 to 3 detected by a synchronous code 2 arearranged in one frame, for example.

FIG. 12 is a block diagram showing a schematic structure of the decoderaccording to the fourth embodiment. This decoder 60 is provided with amultiplex information decoder 64 which decodes the multiplexinformation. This is different from the decoder 40 shown in FIG. 10.Legends same as in FIG. 10 are provided to the other components whichare common to the decoder 40 according to the third embodiment, and theexplanation thereof is omitted. The multiplex information storagesection 260 receives a signal which represents the multiplex informationoutput from the multiplex information decoder 64 so as to rewrite themultiplex information to be stored based on the input signal.

Operations of the decoder 60 which are different from those in the thirdembodiment will only be explained here. A plurality of informationsource data are coded by a system composed of the Reed-Solomon codingcircuit, the frame structure circuit, the energy dispersion circuit andthe interleaver in the multiplexer 110 of the coder 100. Moreover, themultiplex information is coded by the Reed-Solomon coding circuit andthe energy dispersion circuit, and a synchronous code is added to theinformation source data and the multiplex information and they aremultiplex. As a result, the data stream output from the coder of thetransmitter is obtained.

The synchronizer 62, in the same manner as shown in FIG. 6, generates acontrol signal to be input into the multiplex control signal generator250. Moreover, the synchronizer 62 extracts the data stream and the datastream representing the multiplex information from the input signal.

The data stream extracted in the synchronizer 62 is input into the datastream decoder 240 so as to undergo the decoding process. Meanwhile, thedata stream representing the multiplex information in the synchronizer62 is input into the multiplex information decoder 64. Although notshown in this figure, the multiplex information decoder 64 is composedof an energy dispersion signal removal circuit and a Reed-Solomon codeerror correcting circuit. The multiplex information decoder 64 decodesthe multiplex information from the input data stream.

The multiplex information decoded in the multiplex information decoder64 is input as a multiplex information signal into the multiplexinformation storage section 260. The multiplex information storagesection 260 updates the stored multiplex information into multiplexinformation represented by this multiplex information signal. Themultiplex information stored in the multiplex information storagesection 260 is referred to by the multiplex control signal generator250.

The decoding method and the decoder according to the fourth embodimentinitialize the Viterbi decoder 220 in the transmission system whichtransmits and receives data composed of multiplexed data stream at thepoint that the strength of noise mixed in the received signal exceedsthe predetermined value and the data stream having coding rate of lowerror correction ability is changed into the data stream having codingrate of high error correction ability. Further, the multiplex datastream is distributed, and in the case where the multiplex informationis included in the received signal, the multiplex information is decodedso that the decoding form of the respective multiplexed data streams canbe changed dynamically. As a result, in the case where the transmissionsignal where the multiplex form is changed is decoded, the effect of thethird embodiment can be produced.

As explained above, according to the present invention, when thesignal-to-noise ratio monitor detects that the noise exceeds apredetermined value, only then the path metric of the Viterbi decoder isinitialized. For this reason, only in the case where a noise where aninitializing effect can be obtained even after adding deterioration ofthe decoding characteristic due to the initialization occurs, theinitialization is possible. As a result, the optimal decoding with highstability and reliability can be executed in comparison with thedecoding which is executed in the case where a comparatively big noiseis mixed in the received signal.

Furthermore, when the timing that the initialization signal is input isthe point that the data stream having coding rate of low errorcorrection ability is changed into the data stream having coding rate ofhigh error correction ability, only then the Viterbi decoder isinitialized. For this reason, the calculated result of the path metriccalculated by decoding the data stream having coding rate of high errorcorrection ability can be utilized when the next data stream havingcoding rate of low error correction ability is decoded. As a result, thehigh-reliable decoded result which is not much influenced by noise canbe obtained.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A method of decoding time division multiplexed data including aplurality of data streams convolutionally encoded with different codingrates and modulated using any one of BPSK, QPSK, and 8PSK , the methodcomprising: decoding, using a Viterbi decoding algorithm, a first datastream that is included in the time division multiplexed data andmodulated using any one of 8PSK and QPSK; decoding, using the Viterbidecoding algorithm, a second data stream that is included in the timedivision multiplexed data and modulated using any one of QPSK or BPSK;and initializing, after completion of the decoding of the first datastream and before performing the decoding of the second data stream, apath metric that has been calculated before the completion of thedecoding of the first data stream, except when both the first datastream and the second data stream are modulated using QPSK.
 2. Themethod according to claim 1, wherein the path metric is initialized whena coding rate of the second data stream is larger than a coding rate ofthe first data stream.
 3. The method according to claim 1, furthercomprising: measuring a strength of a noise included in the timedivision multiplexed data; and checking whether the strength is equal toor greater than a predetermined value, wherein the path metric isinitialized at the initializing when the strength is equal to or greaterthan a predetermined value.
 4. The method according to claim 1, whereinthe first data stream has a coding rate of low error correction ability,and the second data stream has a coding rate of high error correctionability.
 5. A data receiving system comprising: a Viterbi decoder thatdecodes time division multiplexed data including a plurality of datastreams convolutionally encoded with different coding rates andmodulated using any one of BPSK, QPSK, and 8PSK ; and an initializationsignal generation unit that outputs an initialization signal for a pathmetric to the Viterbi decoder after completion of decoding of a firstdata stream, which is included in the time division multiplexed data andmodulated using any one of 8PSK and QPSK, and before performing decodingof a second data stream, which is included in the time divisionmultiplexed data and modulated using any one of QPSK and BPSK, exceptwhen both the first data stream and the second data stream are modulatedusing QPSK.
 6. The data receiving system according to claim 5, furthercomprising a signal selection, unit that receives the initializationsignal from the initialization signal generation unit, checks whether acoding rate of the second data stream is larger than a coding rate ofthe first data stream, and provides the initialization signal to theViterbi decoder when the coding rate of the second data stream is largerthan the coding rate of the first data stream.
 7. The data receivingsystem according to claim 5, further comprising a distribution unit thatdivides the time division multiplexed data decoded by the Viterbidecoder into a plurality of information corresponding to the pluralityof data streams.
 8. The data receiving system according to claim 5,further comprising a multiplexed information decoding unit that extractsand decodes multiplexed information included in the time divisionmultiplexed data.
 9. The data receiving system according to claim 5,further comprising: a signal-to-noise ratio monitor that measures astrength of a noise included in the time division multiplexed data; anda comparison unit that checks whether the strength is equal to orgreater than a predetermined value and outputs a notification signal tothe initialization signal generation unit when the strength is equal toor greater than the predetermined value, wherein the initializationsignal generation unit outputs the initialization signal upon receivingthe notification signal from the comparison unit.
 10. The data receivingsystem according to claim 9, further comprising a register that storestherein the predetermined value.
 11. The data receiving system accordingto claim 5, wherein the first data stream has a coding rate of low errorcorrection ability, and the second data stream has a coding rate of ahigh error correction ability.
 12. A decoder comprising: a Viterbidecoder that decodes time division multiplexed data including aplurality of data streams convolutionally encoded with different codingrates and modulated using any one of BPSK, QPSK, and 8PSK ; and aninitialization signal generation unit that outputs an initializationsignal for a path metric to the Viterbi decoder after completion ofdecoding of a first data stream, which is included in the time divisionmultiplexed data and modulated using any one of 8PSK and QPSK, andbefore performing decoding of a second data stream, which is included inthe time division multiplexed data and modulated using any one of QPSKand BPSK, except when both of the first data stream and the second datastream are modulated using QPSK.
 13. The decoder according to claim 12,further comprising: a signal-to-noise ratio monitor that measures astrength of a noise included in the time division multiplexed data; anda comparison unit that checks whether the strength is equal to orgreater than a predetermined value and outputs a notification signal tothe initialization signal generation unit when the strength is equal toor greater than the predetermined value, wherein the initializationsignal generation unit outputs the initialization signal upon receivingthe notification signal from the comparison unit.
 14. The decoderaccording to claim 12, wherein the first data stream has a coding rateof low error correction ability, and the second data stream has a codingrate of high error correction ability.
 15. A data transmitting andreceiving system comprising: a transmitting unit that transmits timedivision multiplexed data including a plurality of data streamsconvolutionally encoded with different coding rates and modulated usingany one of BPSK, QPSK, and 8PSK ; and a receiving unit that receives anddecodes the time division multiplexed data and includes: a Viterbidecoder that decodes the time division multiplexed data; asignal-to-noise ratio monitor that detects a strength of a noiseincluded in the time division multiplexed data; and an initializationsignal generating unit that outputs, based on the strength of the noise,an initialization signal for a path metric to the Viterbi decoder aftercompletion of decoding of a first data stream, which is included in thetime division multiplexed data and modulated using any one of 8PSK andQPSK, and before performing decoding of a second data stream, which isincluded in the time division multiplexed data and modulated using anyone of QPSK and BPSK, except when both of the first data stream and thesecond data stream are modulated using QPSK.
 16. The data transmittingand receiving system according to claim 15, wherein the initializationsignal is output to the Viterbi decoder when a coding rate of the seconddata stream is larger than a coding rate of the first data stream.
 17. Amethod of decoding time division multiplexed data including a pluralityof data streams convolutionally encoded and modulated using any one ofBPSK, QPSK, and 8PSK the method comprising: decoding, using a Viterbidecoding algorithm, a first data stream that is included in the timedivision multiplexed data and modulated using any one of 8PSK and QPSK;decoding, using the Viterbi decoding algorithm, a second data streamthat is included in the time division multiplexed data and modulatedusing any one of QPSK and BPSK; and initializing after completion of thedecoding of the first data stream and before performing the decoding ofthe second data stream, a path metric that has been calculated beforethe completion of the decoding of the first data stream, except whenboth of the first data stream and the second data stream are modulatedusing QPSK.
 18. The method according to claim 17, further comprising:measuring a strength of a noise included in the time divisionmultiplexed data; and checking whether the strength is equal to orgreater than a predetermined value, wherein the path metric isinitialized at the initializing when the strength is equal to or greaterthan a predetermined value.
 19. The method according to claim 17,wherein the first data stream has a coding rate of low error correctionability, and the second data stream has a coding rate of high errorcorrection ability.
 20. A data receiving system comprising: a Viterbidecoder that decodes time division multiplexed data including asynchronous code, multiplex information, and a plurality of data streamsconvolutionally encoded with different coding rates and modulated usingany one of BPSK, QPSK, and 8PSK ; and an initialization signalgeneration unit that outputs an initialization signal for a path metricto the Viterbi decoder after completion of decoding of a first datastream, which is included in the time division multiplexed data andmodulated using any one of 8PSK and QPSK, and before performing decodingof a second data stream, which is included in the time divisionmultiplexed data and modulated using any one of QPSK and BPSK, exceptwhen both of the first data stream and the second data stream aremodulated using QPSK.
 21. The data receiving system according to claim20, further comprising a synchronizer that outputs a control signal tothe initialization signal generation unit upon detecting the synchronouscode.
 22. The data receiving system according to claim 20, furthercomprising: a signal-to-noise ratio monitor that measures a strength ofa noise included in the time division multiplexed data; and a comparisonunit that checks whether the strength is equal to or greater than apredetermined value and outputs a notification signal to theinitialization signal generation unit when the strength is equal to orgreater than the predetermined value, wherein the initialization signalgeneration unit outputs the initialization signal upon receiving thenotification signal from the comparison unit.
 23. The data receivingsystem according to claim 20, wherein the first data stream has a codingrate of low error correction ability, and the second data stream has acoding rate of high error correction ability.
 24. A data transmittingand receiving system comprising: a transmitting unit that transmits timedivision multiplexed data including a plurality of data streamsconvolutionally encoded with different coding rates and modulated usingany one of BPSK, QPSK, and 8PSK; and a receiving unit that receives anddecodes the time division multiplexed data and includes, a Viterbidecoder which that decodes the time division multiplexed data; and aninitialization signal generation unit that outputs an initializationsignal for a path metric to the Viterbi decoder after completion ofdecoding of a first data stream, which is included in the time divisionmultiplexed data and modulated using any one of 8PSK and QPSK, andbefore performing decoding of a second data stream, which is included inthe time division multiplexed data and modulated using any one of QPSKand BPSK, except when both of the first data stream and the second datastream are modulated using QPSK.
 25. The data transmitting and receivingsystem according to claim 24, further comprising: a signal-to-noiseratio monitor that measures a strength of a noise included in the timedivision multiplexed data; and a comparison unit that checks whether thestrength is equal to or greater than a predetermined value and outputs anotification signal to the initialization signal generation unit whenthe strength is equal to or greater than the predetermined value,wherein the initialization signal generation unit outputs theinitialization signal upon receiving the notification signal from thecomparison unit.
 26. A decoder comprising: a Viterbi decoder thatdecodes time division multiplexed data including a synchronous code,multiplex information, and a plurality of data streams convolutionallyencoded with different coding rates and modulated using any one of BPSK,QPSK, and 8PSK ; and an initialization signal generation unit thatoutputs an initialization signal for a path metric to the Viterbidecoder after completion of decoding of a first data stream, which isincluded in the time division multiplexed data and modulated using anyone of 8PSK and QPSK, and before performing decoding of a second datastream, which is included in the time division multiplexed data andmodulated using any one of QPSK and BPSK, except when both of the firstdata stream and the second data stream are modulated using QPSK.
 27. Thedata decoder according to claim 26, further comprising a synchronizerthat outputs a control signal to the initialization signal generationunit upon detecting the synchronous code.
 28. The decoder according toclaim 26, further comprising: a signal-to-noise ratio monitor thatmeasures a strength of a noise included in the time division multiplexeddata; and a comparison unit that checks whether the strength is equal toor greater than a predetermined value and outputs a notification signalto the initialization signal generation unit when is equal to or greaterthan the predetermined value, wherein the initialization signalgeneration unit outputs the initialization signal upon receiving thenotification signal from the comparison unit.
 29. The decoder accordingto claim 26, wherein the first data stream has a coding rate of a lowerror correction ability, and the second data stream has a coding rateof high error correction ability.